Raid1 makes a mirror of a drive. So, if something fails, such as a file or folder is removed or permissions changed, then it would affect the second drive.
I'd go with WHS's suggestion and make frequent images, perhaps once a week.
That way, if something fails, the image is your backup.
Doing it that way, the image will not include your free space. So if you have 400GB of data on your primary disk, the image file would be 400GB (unless you use compression then its smaller of course).